Thermal transfer systems have been developed in recent years to cater to the demand of instant access color hard copies of pictures generated electronically by solid state video cameras and the like. Thermal media also find application in color proofing for graphic arts. Thermal transfer imaging involves imagewise transfer of colourants from a donor to a receptor sheet under the action of heat, the donor and receptor sheets being maintained in intimate face-to-face contact, generally through the application of pressure or vacuum. This type of imaging has become increasingly popular because thermal media offer the advantages that they can be handled in daylight, do not require wet processing, generate no polluting effluent and are therefore compatible with the office or home environment.
A color scanner is used to achieve color separation of a picture into its red, green and blue constituents (or its cyan, magenta, and yellow substituents, with an optionally prefered black component). These color separated images are converted into electrical signals which can be operated upon and manipulated. In a known system, these signals are then fed to a thermal printer to drive the thermal print heads. To obtain the print, a colourant donor element, e.g. in the form of a ribbon, with sequentially printed yellow, magenta and cyan color blocks is placed in face-to-face contact with a receiving element and the resulting composite passed between a thermal print head and a platen roller. Imagewise heating is provided by the thermal print head which consists of a number of resistive elements deposited by a thin film process onto a substrate, e.g. alumina and arranged in a linear array. Each approximately square element of the thermal print head is independently addressable by virtue of multiple input lines and logic circuitry on the head. Printing is carried out by energising the head with electrical signals corresponding to the image. The process is repeated for each of the primary colours.
Thermal transfer processes using print heads suffer the drawbacks of low resolution due to the difficulty and cost of fabricating smaller and smaller heating elements, easy soiling of the head and shortened life due to constant contact with the donor element. Under pressure and at the elevated operating temperatures, the head tends to stick to the base of the donor sheet.
Another method of obtaining color hard copies from electronic signals is to use a coherent light source such as a laser to apply imagewise heating to a donor element containing a radiation absorber. Upon exposure to a laser, the light absorber converts light energy into thermal energy and transfers the heat to the matrix in the immediate vicinity, thus increasing the mobility of the colourant for transfer to the receiving element. The light absorbing material may be present in a layer beneath the colourant and/or in admixture with it. The laser beam is modulated by electronic signals which are representative of the shape and color of the original image causing localised heating and imagewise transfer of the three primary colors or complemetary colors (and optionally black) in sequence to reconstruct the original image. Laser-induced thermal colourant transfer is advantageous in that it is a non-impact printing method and has high resolution capabilities. Colors other than these conventional imaging colors may be used such as fluorescent colors, metallics. white, grey, and custom colors.
Examples of thermal transfer media are disclosed in GB-A-1385533, GB-A-2083726; EP-A-403932, EP-A-403933, EP-A-403934, EP-A-404042, EP-A-405219, EP-A-405296, EP-A-407744, EP-A-407907, EP-A-408891, EP-A-408908; U.S. Pat. No. 3,787,210, U.S. Pat. No. 3,946,389, U.S. Pat. No. 4.541.830, U.S. Pat. No. 4,602,263, U.S. Pat. No. 4,788,128, U.S. Pat. No. 4,904,572, U.S. Pat. No. 4,912,083, U.S. Pat. No. 4,942,141, U.S. Pat. No. 4.948.776, U.S. Pat. No. 4,948,777, U.S. Pat. No. 4.948.778, U.S. Pat. No. 4,950,639, U.S. Pat. No. 4,950,640, U.S. Pat. No. 4,952,552, U.S. Pat. No. 4,973,572; WO88/04237; JP-21075292, JP-30043294, JP-51088016, JP-56082293, JP-63319191 and JP-63319192.
A slightly different arrangement in which the infrared absorber is situated in the receiving element rather than in the donor element is disclosed in JP-04278390, JP-04153087 and PCT/GB92/01489.
A wide variety of materials suggested as radiation absorbers include carbon black, as disclosed in GB-A-2083726, and a wide range of visible and infrared absorbing dyes such as phthalocyanines (U.S. Pat. No. 4,788,128), ferrous complexes (U.S. Pat. No. 4,912,083), squarylium dyes (U.S. Pat. No. 4,942,141), chalcogenopyrylo-arylidene dyes (U.S. Pat. No. 4,948,776), bis(chalcogenopyrylo)polymethine dyes (U.S. Pat. No. 4,948,777), oxoindolizine dyes (U.S. Pat. No. 4.948.778), bis(aminoaryl) polymethine dyes (U.S. Pat. No. 4,950,639), tetraaryl polymethine dyes, merocyanine dyes (U.S. Pat. No. 4,950,640), anthraquinone and naphthoquinone derived dyes (U.S. Pat. No. 4,952,552), cyanine dyes (U.S. Pat. No. 4,973,572 and JP-02173291), bridged cyanine dyes (JP-04169289), trinuclear cyanine dyes (EP-A-403933), pyrrocoline dyes (JP-04161382 and JP-04169290), oxonol dyes (EP-A-403934 and U.S. Pat. No. 5,035,977), indene-bridged polymethine dyes (EP-A-407744), nickeldithiolene dyes (EP-A-408908), chromylium squaraine dyes (WO92/09661 and EP-A-511381), thiopyrylium squaraine dyes (U.S. Pat. No. 5,019,549), thiochromylium squaraine dyes (JP-04153086), polyisothianaphthene dyes (JP-022064), indoaniline and azomethine dyes (JP-04173290), indoaniline methide dyes (JP-04189590), tetraarylaminium radical cation dyes (WO90/12342) and metallized quinoline indoaniline dyes (JP-04153086). Squarylium dyes or squaraines have been disclosed previously as infrared absorbers in thermal transfer media, e.g. U.S. Pat. No. 4,942,141, U.S. Pat. No. 5,019,549, EP-A-511381, JP-04153086, JP-63319191 and JP-63319192.
A problem that can arise with laser addressed thermal dye transfer is co-transfer of the radiation absorber with the colourant. Since most dyes absorb to some extent in the visible region of the spectrum, any contamination of the transferred image can result in an inaccurate color rendition. There is a continuing need for near-infrared absorbing materials that are colourless or show minimal absorption in the visible spectrum.
EP 0478052 discloses infra-red sensitive liquid-crystalline polyesters for optical data storage purposes in which a dye is covalently bonded to a liquid crystalline polyester. Suitable dyes include squarilium dyes having dihydroperimidine terminal groups. This class of dyes is also disclosed in J.Chem. Soc., Chem.Commun. 1993 pages 452-454. There is no disclosure of the use of these dyes in thermal dye transfer.